Active noise-cancelling muffler systems used with internal combustion engines typically include means for monitoring selected parameters of the exhaust system and gas flow; and using the parameters, developing a noise-cancelling acoustic waveform. The "counter-acoustic wave" typically is formed first as an electrical waveform generated by a controller. The controller may be a computer or chip driver connected to an amplifier for a transducer that generates the cancelling signal. The cancelling wave and the exhaust gas energy continuously subtractively combine to effect the desired noise reduction.
In active noise-cancelling muffler systems, it is necessary to control the noise-cancelling acoustic signal both spatially and temporally so that the negative-going pulses of the cancelling wave coincide with the positive-going pulses of the exhaust gasses. The prior art teaches a variety of control strategies, using various physical structures to contain the transducer and launch the counter-acoustic wave. However, the physical design of the systems as well as the efficiency of the control signal leave much to be desired in terms of cost and reliability.
The controller requires accurate information as to the upstream exhaust gas reference pressure in order to generate a useful transducer input signal. One problem with many such systems of the prior art is that acoustic or mechanical coupling occurs between the counter-acoustic wave generator and the exhaust system of the IC engine. Readings of the exhaust gas reference pressure that are perturbed by mechanical or acoustical coupling from the noise cancelling apparatus, complicate the controller's function by requiring more complex and time-consuming computations to compensate for the perturbations. If the actual on-going reference gas pressure fluctuations is substantially obsecuted by such perturbation, the functionality of the system may be defeated altogether.
A continuously effective noise reduction system requires constant adjusting of the cancelling waveform to changing conditions which include exhaust temperature, frequency and amplitude. Ideally, despite the changing conditions, the exhaust gas acoustic energy is driven by the cancelling waveform toward zero at all times.
The degree of success in achieving full cancellation depends in part on continuously measuring the actual noise reduction occurring at the exhaust pipe outlet. The measurement of noise reduction is critical in determining controller inputs that will cause the transducer to continuously drive the exhaust gas noise to zero.
In most prior art active control noise-cancelling vehicular muffler systems, because the cancelling waveform and the exhaust noise waveform are acoustically and mechanically coupled in the same pipe, it is straightforward to measure the actual noise reduction simply by placing a single sensor at the common outlet. If, on the other hand, the noise-cancelling generator and the exhaust pipe are physically decoupled, the prior art expedient for measuring actual noise reduction is not available. Further, it is not practical for cost and reliability reasons to place a pickup microphone in the space beyond the exhaust pipe outlet to measure the actual reduced-noise exhaust gas amplitude. A practical measure of the spatial and temporal components of the noise-reduced exhaust gas pressure for use with decoupled systems is needed.